Gene regulated by ectopic expression of Sage in the entire Drosophila embryo. Drosophila melanogaster

FoxA transcription factors play major roles in organ-specific gene expression. How FoxA proteins achieve specificity is unclear, given their broad expression patterns and requirements in multiple cell types. Here, we characterize Sage, a basic helix-loop-helix (bHLH) transcription factor expressed exclusively in the Drosophila salivary gland (SG). We identify Sage targets and show that not only are both Sage and the single Drosophila FoxA protein, Fork head (Fkh), required for expression of these genes, but coexpression of Sage and Fkh is sufficient to drive target gene expression in multiple other cell types. Sage and Fkh drive expression of the bZip transcription factor Senseless (Sens), which boosts expression of Sage/Fkh targets. Importantly, Sage, Fkh and Sens colocalize on salivary gland polytene chromosomes. Thus, Fkh drives cell-type specific gene expression as part of a tissue-specific transcription module that includes Sage and Sens, providing a new paradigm for how mammalian FoxA proteins acheive specificity. Overall design: Three wild type (reference) samples were obtained from the Oregon R strain, age matched and treated the same as the three experimental samples isolated from Stages 11-16 embryos expressing UAS-Sage controlled by the tub-Gal4 driver.

FoxA转录因子（FoxA transcription factors）在器官特异性基因表达中发挥核心调控作用。鉴于FoxA蛋白广泛的表达谱以及在多种细胞类型中的功能需求，其如何实现靶基因的特异性调控仍未阐明。本研究对Sage——一种仅在果蝇（Drosophila）唾液腺（salivary gland, SG）中表达的碱性螺旋-环-螺旋（basic helix-loop-helix, bHLH）转录因子——进行了系统表征。我们鉴定了Sage的下游靶基因，并证实：不仅Sage与果蝇唯一的FoxA蛋白叉头蛋白（Fork head, Fkh）均为这些靶基因的正常表达所必需，且二者的共表达足以在多种异源细胞类型中激活靶基因的转录。Sage与Fkh可诱导碱性亮氨酸拉链（basic leucine zipper, bZip）转录因子感觉神经元蛋白（Senseless, Sens）的表达，而Sens能够进一步增强Sage/Fkh靶基因的转录水平。尤为关键的是，Sage、Fkh与Sens共定位在果蝇唾液腺的多线染色体（polytene chromosomes）上。综上，Fkh作为包含Sage与Sens的组织特异性转录调控模块的组分，介导细胞类型特异性基因表达，这为解析哺乳动物FoxA蛋白如何实现调控特异性提供了全新的研究范式。实验设计：本研究获取3份野生型（wild type, 参照）样本，均来自俄勒冈R品系（Oregon R strain），与3份实验样本保持年龄匹配且处理方式完全一致；实验样本取自发育第11-16期的胚胎，该胚胎通过tub-Gal4驱动系（tub-Gal4 driver）表达UAS-Sage。